Whisking--the rhythmic whisker movements that rats perform while exploring their environment--is emerging as an exceptionally promising model for investigating mechanisms that control of voluntary movements. Exploitation of this model has been limited by lack of data on the pathways and mechanisms responsible for this important motor behavior. In the current funding period we made the exciting discovery that serotonergic pre-motoneurons are both necessary and sufficient to generate rhythmic firing in the facial motoneurons that control whisking. Thus, these serotonergic neurons are a critical component of a central pattern generator (CPG) for rhythmic whisking. We also showed that this serotonergic CPG receives dense inputs from the whisker representation of the motor cortex (wMCx). Finally, we demonstrated that--contrary to prevailing dogma--lesions of wMCx significantly affect whisking kinematics and rhythmicity. Our central hypothesis is based on these convergent findings. It states that wMCx modulates the activity of the serotonergic CPG that regulates the frequency of exploratory whisking, a function critical for sensorimotor integration and discrimination abilities. From this central hypothesis we formulate and test three novel hypotheses directed at a complete understanding of the neural mechanisms underlying this important motor behavior: Hypothesis I: Facial motoneurons that control whisking (whisking motoneurons) are synchronized by reciprocal electrical synapses. Hypothesis II: Serotonin released from the CPG dynamically regulates whisking frequency. Hypothesis III: wMCx modulates whisking frequency by its action on the serotonergic CPG.